X-ray intensifying screens are well known in the art and generally comprise, in order, (a) a support, (b) an active layer comprising a fluorescent phosphor dispersed in a suitable binder, and (c) a protective topcoat or abrasion layer coated over the active layer to protect the screen during use. In operation, the intensifying screen absorbs impinging X-rays and emits energy having a wavelength that is readily captured by photographic silver halide X-ray film. A reflective layer, such as TiO.sub.2 dispersed in a suitable binder, is typically also present in the intensifying screen, on either side of the support or incorporated directly into the support, to maximize the proportion of energy emitted by the phosphor that will impinge on the X-ray film.
Typical X-ray films are constructed of a substrate (e.g., a polyethylene terephthalate film) that bears gelatino-silver halide emulsion coated on both sides of the substrate. In use, the film is placed in a book-type cassette, with an intensifying screen in intimate contact with both film surfaces. The cassette is placed in proximity with a patient, in the area under examination, and the patient is exposed to X-rays. The film is then removed and processed. Much of the handling is done in the dark to protect the film from undesired exposure. Large hospitals, which handle many X-rays daily, generally use automatic changer and processing devices in which unexposed film is successively fed into position between a pair of X-ray intensifying screens, exposed, and automatically unloaded.
As mentioned above, conventional X-ray intensifying screens have a protective topcoat that is intended to protect the screen, which is relatively expensive, from damage. The ideal topcoat will possess a number of desired properties, including: good adhesion to the phosphor-containing active layer; abrasion and scratch resistance to foreign particles and edges of X-ray film; resistance to cracking/crazing caused by flexing of the screen in use; a low coefficient of friction over the relative humidity range of 10-80%; resistance to degradation (yellowing) that would effect image quality; the ability to withstand damage due to handling abuse; be relatively free of static build-up often caused by sliding contact between the X-ray film and intensifying screen, which can lead to static discharge resulting in blemishes on the X-ray film; and relatively impermeable to fluids normally found in a hospital environment. Durability of the topcoat is especially critical when the intensifying screen is used in an automatic changer, where it is subject to rough treatment.
Conventional topcoat materials, such as cellulose acetate and similar polymers, have not provided desired abrasion and stain resistance, and tend to delaminate from the fluorescent phosphor layer, especially in exacting uses such as in automatic changer systems. Over the years a variety of polymers have been proposed as replacement candidates for cellulose acetate in an effort to provide the balance of desired properties noted above. For example, a fluoroester topcoat was recently proposed in U.S. Pat. No. 4,491,620 of Joiner. While the fluoroester resin provides improved adhesion and stain resistance, it is prone to static build-up and abrasive wear. Thus, the art has continued to seek an improved topcoat for X-ray intensifying screens.